Aerodynamics:

The Law of Reactions - Newton's Third Law

For every action there is an equal but opposite reaction.

The above is the law of reactions, sometimes referred to as Newton's third law.

You have probably swung something on a string at some point in your life. When you did you realized that you had to pull on the string with a force. That force is the black vector in the picture to the left, and it is called the force of centripetal acceleration. If you think back to your experience swinging things on strings, you also recall that the object on the string pulled on your hand too. This phenomenon is what Newton was talking about when he said that for every action there is an equal but opposite reaction. When Newton said action, he meant force application. In other words Newton tells us that when we apply a force there will be another force that appears but in the opposite direction. The force opposite to centripetal force is known as centrifugal force.

In simple terms Newton's third law tells us that if we push on something, that thing will push back on us. Jet engines produce thrust by pushing chemicals backward. The reaction is what we call thrust, and it is forward. (Note: the chemicals referred to include hydrocarbons, water, and air molecules.) Propeller aircraft produce thrust by pushing (accelerating) air backwards. Thrust is the reaction.

Is Centrifugal Force Real?

Somewhere along the line some people have been told that centrifugal force is "not real." But, it is as real as lift, and thrust, which are also reaction forces. Perhaps the confusion comes from the fact that there is no equation for centrifugal force. Centrifugal is simply equal to centripetal, but 180 degrees in the opposite direction. The 180 degrees opposite is captured mathematically by changing the sign. I.E. if centripetal force is positive then centrifugal is negative and vice versa.